Preheat oven to 350 degrees and line a cupcake pan with paper liners.
In a medium bowl, combine flour, baking powder, cinnamon and salt. Set aside.
In a stand mixer or in a large bowl and a hand mixer, cream together butter and brown sugar for about 4 minutes, until light and fluffy on medium/medium high speed.
Add the egg, mixing well and scraping the sides of the bowl.
In a small bowl or pourable measuring cup, whisk together milk and vanilla. Set aside.
Starting with the dry ingredients, add about ⅓ to the butter mixture and beat until combined.
Add half of the milk and mix well. Continue alternating dry and wet ingredients, ending with the dry ingredients.
In a small bowl, combine the 2 tablespoons of sugar and 2 teaspoons of cinnamon.
Pour batter into each cupcake liner filling about ⅓ full. Sprinkle about ¼ teaspoon of cinnamon sugar over each cupcake and swirl it into the batter using a knife. Top each cupcake with the remaining batter evenly. Sprinkle another ¼ teaspoon of cinnamon sugar over each cupcake and swirl into the batter using a knife.
Bake for about 16 minutes or until a toothpick inserted in the middle of the cupcake comes out clean.
Let cool in the pan for about 5 minutes and then transfer to a wire rack to cool completely before frosting with the buttercream.